The present invention concerns a reference electrode for electrochemical measurements.
It is known to provide reference electrodes in electrochemical measuring circuits as an electrical reference point in a medium to be investigated. Reference electrodes are reversible nonpolarizable half-cells which form the most constant possible potential substantially independent of the components of the medium to be investigated.
The primary reference electrode in electrochemical measuring technology is the standard hydrogen electrode, whose potential is by definition set equal to zero at all temperatures. In practice, however, one uses almost exclusively electrodes of a second type as reference electrodes since these are easier to handle. They consist essentially of a metal which is coated with a layer consisting of a poorly soluble compound of this metal and is immersed in a solution containing a soluble compound with the anion present in the layer [F. Oehme: Ion-Selective Electrodes, Dr. Alfred Huthig Verlag, Heidelberg 1986, p. 67]. Most widespread are reference electrodes of the type Ag/AgCl, Cl, and Hg/Hg2Cl2. The standard potentials of these reference electrodes are precisely known so that the potentials measured therewith can be easily converted to the potential of the standard hydrogen electrode [K. Schwabe: Physical Chemistry. Volume 2: Electrochemistry. Akademie Verlag, Berlin 1975, p. 177]. Since the mentioned reference electrodes are universally applicable and exhibit high potential constancy, other metallic electrode materials have only slight importance in electrochemical reference electrodes compared to the mentioned metals silver and mercury.
From Lambrechts, M. et al., Biosensors: Microelectrochemical Devices, Katholieke Universiteit Leuven, Institute of Physics Publishing, Bristol, Philadelphia and New York (1992), page 165, one already recognizes a reference electrode of the type mentioned above which exhibits on a substrate of semiconductor material a layer of metallic silver and, on the latter, a layer of silver chloride. However, this reference electrode has the disadvantage that the silver chloride is photosensitive and can be reduced to metallic silver through the action of light. In order to avoid destruction of the reference electrode through the action of light, suitable protective measures are consequently required. Combined with the prior-art reference electrode is a working electrode of gold, which is connected with the reference electrode to form an electrochemical measuring circuit. Working and reference electrodes are arranged on a common substrate. A disadvantage of this electrode arrangement consists, however, in the fact that the reference and working electrodes are produced from different metallic materials, namely from silver and gold, which requires the application of additional lithographic masks in the production of the reference electrode using planar technology.
The object therefore is to create a reference electrode of the above-noted type in which the previously mentioned disadvantages are avoided.
The solution to this problem lies in the fact that the reference electrode has an electrode component exhibiting metallic palladium, in the fact that this electrode component is covered by a layer which consists of or contains a palladium compound poorly soluble in aqueous media, and in the fact that a reference electrolyte containing anions of this palladium compound in dissolved form is positioned thereon.
A working electrode of palladium can then be combined with the electrode component in an electrochemical measuring circuit such that the reference and working electrodes can be produced from the same metallic starting material. Thus, for example, the electrode component of the reference electrode can be a palladium wire coated with a compound of palladium that is poorly soluble in aqueous media and immersed in an aqueous or gel-like medium containing anions of the palladium compound. An uncoated palladium wire can then be provided as the working electrode. Additionally advantageous is the fact that the reference electrode according to the present invention is highly insensitive to light. Consequently, lightproof shielding of the reference electrode can be foregone.
To be sure, the use of palladium as sensor material is already known from Lundstrxc3x6m, I., et al., Hydrogen-sensitive MOS field effect transistor, Appl. Phys. Lett. 26 (1975), p.55-67, for a different kind of hydrogen-sensitive gas sensor. If, in a MOS field effect transistor, the metal on the gate insulator SiO2 is replaced by a thin layer ( less than 100 nm) of palladium, hydrogen atoms diffuse from the surroundings through this layer to the Pd/SiO2 phase interface, through which the work function in the palladium layer is changed and a measurable shift occurs in the threshold voltage of the field effect transistor. The prior-art hydrogen-sensitive gas sensor, however, exhibits an entirely different configuration compared to the reference electrode of the present invention and does not serve in obtaining a reference potential.
It has also already been attempted to use palladium in producing industrial hydrogen electrodes as type-one electrodes for pH measurement in hydrofluoric acid-containing etching baths, these electrodes being based on the idea that palladium can take up hydrogen in larger amounts [Jasinski, R., A Palladium Hydride pH Electrode for Use in Buffered Fluoride Etch Solutions, J. Electrochem. Soc. 121 (1974), p. 1579-1584]. However, the service lives of such electrodes are short. For this reason, they have not attained any technical importance up to now [Galster, H., pH measurement, VCH Verlagsgesellschaft, Weinheim (1990), p.69; Shao, M. et al., pH measurements based on a palladium electrode, Electroanalysis 6 (1994), p. 245-249].
The palladium/hydrogen system is also specified in U.S. Pat. No. 4,404,065 as reference system in miniaturizable design, e.g., for coulometric sensors in flow processes or for amperometric sensors of thin-layer design. In spite of the advantage that this reference system is applicable up to high pressures ( greater than 4 Mpa), its applicability is substantially limited as a result of the fact that pronounced pH sensitivity occurs.
Also already known from GB-A-2 023 846 is a reference electrode with an electrode component of metallic palladium which is covered by a layer of palladium oxide standing in contact with the reference electrolyte. This reference electrode also exhibits a pronounced dependence of its electrode potential on the pH value of the reference electrolyte.
Also already described in U.S. Pat. No. 4,536,274 is a sensor for transcutaneous measurement of the CO2 content in blood. The sensor exhibits a measuring electrode with an electrode component of metallic palladium, which is covered by a layer of palladium oxide. Via the measuring electrode, the CO2 content is indirectly determined through measurement of the pH value. A silver/silver-chloride electrode is assigned as a reference electrode to the measuring electrode. The electric potential at the palladium/palladium-oxide electrode varies as a function of the pH value of the electrolyte standing in contact with the electrode while the potential of the reference electrode remains constant independent of the pH value.
In an especially advantageous embodiment of the present invention, the palladium electrode component is formed as a layer positioned on a substrate formed of semiconductor material, the electrode component particularly being applied to the substrate using a semiconductor-technology production process. The reference electrode can then exhibit especially small dimensions. Beyond this, in producing the reference electrode, additional structures can also be introduced into or applied onto the substrate via the semiconductor-technology production process, for example, structures for an evaluation device for processing a measurement signal, a working electrode, a sensor, and/or similar components. Here, an applied metallization for producing the palladium electrode component on the substrate or the layers found thereon can also be simultaneously utilized in forming a palladium structure for an additional component. The layer containing the poorly soluble palladium salt can be very effectively and economically applied since a large number of electrode structures simultaneously prepared in one production step can be located on one wafer.
The palladium electrode component can be applied especially advantageously via the process known from DE 196 41 777 A1. Here, a structure is produced on the substrate for the palladium electrode component formed of a material with predetermined adhesive properties for metallic palladium. One subsequently undertakes palladium metallization of the surface of the configuration exhibiting the structure in which the palladium metallization remains adhering only to the structure for the palladium electrode component. In this way, the palladium electrode component can be applied to the substrate or the structure located thereon without the use of an additional photolithographic varnish mask. As a result, it is particularly possible to combine the process steps required for the production of the reference electrode in one CMOS process.
In another embodiment of the present invention, the palladium electrode component is formed as a layer which is located on a substrate consisting of ceramic, glass, glass ceramic, printed-circuit-board material, polymeric material, or similar dielectric material and which is particularly applied to the substrate via thick-film technology. The reference electrode can then be produced especially economically, for example, using screen or stencil printing.
It is advantageous if the electrode component contains metallic palladium embedded in finely distributed form in a binder of polymeric material or cermet. In manufacturing the reference electrode, the palladium can be applied to the substrate in a pasty or liquid medium, for example, via screen or stencil printing. The medium can subsequently be hardened, dried, or solidified.
The poorly soluble palladium compound can contain palladium iodide, palladium bromide, and/or palladium acetate. The palladium compound can then be produced simply and economically via electrochemical methods. Here, it is even possible for the layer exhibiting the poorly soluble palladium compound to be deposited or produced electrolytically and/or through a chemical precipitation process in situ on the palladium electrode component.
In an advantageous embodiment of the present invention, anions of the poorly soluble palladium compound are contained in an ionogenic solution and/or a solidified gel. Here, the ion-containing solution and/or the solidified gel can possibly be enclosed between an ion-permeable membrane and the metallic palladium layer.
In an advantageous embodiment of the present invention, the reference electrode is integrated on a semiconductor chip or in a hybrid-structured microsystem. The microsystem, of for example, can exhibit a substrate on which the reference electrode and further components such as, e.g., an evaluation device and/or an ion-selective working electrode can be positioned. Here, the individual components can be mounted on the substrate using in themselves known microtechnological methods. Overall, one consequently obtains a compactly structured measuring device.
The electrode component and/or the layer exhibiting the palladium compound poorly soluble in aqueous media can have a ring-shaped or disk-shaped design or exhibit a rectangular shape. Use of the present invention is not limited to a planar reference electrode. A cylindrical reference electrode can also be advantageously produced by immersing a palladium wire coated with a palladium compound poorly soluble in aqueous media in an aqueous or gel-like medium containing anions of the palladium compound.
It is especially advantageous if the reference electrode is part of an electrochemical measuring system with at least one working electrode located on the substrate and formed especially of palladium and/or part of an ion-sensitive field effect transistor and if the working electrode and/or the field effect transistor is preferably positioned on the front side of the substrate opposite the palladium electrode component. As a result, a particularly compactly structured electrochemical measuring device is obtained in which the reference and the working electrode and/or the field effect transistor are located on the same substrate. Since the working electrode can be formed of palladium just like the electrode component, the working electrode and the electrode component can be applied to the substrate and/or the layers located thereon in a single operation. The electrochemical reference system can then be produced especially economically.
Appropriately, at least one temperature sensor is positioned on the substrate. Any influence of the ambient temperature and/or the temperature of the medium to be investigated on the reference potential of the reference electrode can then compensated for or otherwise taken into consideration.
The reference system of the present invention is preferably employed as a reference electrode in an amperometric sensor. For use as a reference electrode in a potentiometric measuring circuit, the reference electrolyte containing the anions of the palladium compound in dissolved form can be covered with a membrane or surface layer perforated at only one point, with precisely this point being closed off by a diaphragm conventionally used for electrochemical measurements, for example, by a porous membrane or a cation-exchange membrane, while remaining permeable to an electrolytic compound. The membrane is preferably chemically inert, electrically insulating, and/or liquid-tight.
Further advantageous embodiments of the present invention are described in the subclaims.